1. Field of the Invention
The present invention relates to a pneumatic tire having a tread pattern including a plurality of main grooves extending along a tire circumferential direction, and a shoulder land portion constructed by a rib or block row.
2. Description of the Related Art
Various tread patterns are formed in a tread surface of a pneumatic tire in correspondence to a demanded tire performance and a used condition. Since there is a case that a cut called a sipe is formed in a land portion of the tread surface, and it is possible to seize on a road surface based on an edge effect of the sipe, it is possible to enhance a turning performance at a time of cornering and a braking performance at a time of braking, even on a wet road surface and a dry road surface, in addition to an ice and snow road surface.
Japanese Unexamined Patent Publication No. 2007-153104 describes a pneumatic tire in which a circumferential sipe is provided in a block, and a forming density of the circumferential sipe is made higher in a vehicle outer side than in a vehicle inner side. This is structured such as to enhance an edge effect in a lateral direction in the vehicle outer side at a time of cornering, thereby achieving an improvement of the turning performance. However, as a result of research work of the present inventor, it is known that there is a tendency that a ground area cannot be sufficiently obtained in the vehicle inner side at a time of cornering according to the structure mentioned above, and there is room for improvement of the turning performance.
Japanese Unexamined Patent Publication No. 7-257114 describes a pneumatic tire in which a tread pattern is formed bilaterally asymmetrically, an exhaust groove is provided in a block in a vehicle inner side, and an exhaust groove is not provided in a block in a vehicle outer side. The exhaust groove extends along a tire circumferential direction, and has a function of communicating sipes in a width direction with each other so as to lower a noise. In the structure mentioned above, since the circumferential sipe is not formed in the vehicle outer side, there is a problem that the edge effect in the lateral direction at a time of cornering tends to come short and the turning performance becomes insufficient.
Japanese Unexamined Patent Publication No. 2006-192929 describes a pneumatic tire in which a tread pattern is formed bilaterally asymmetrically, and the number of the sipes in the width direction provided in the block in the vehicle inner side is made larger than the number of the sipes in the width direction provided in the block in the vehicle outer side. However, this tire structure is provided for improving a snow steering stability, and does not suggest any solving means for securing the ground area in the vehicle inner side at a time of cornering, and further for enhancing the turning performance.